Some long-haul lightwave communication systems rely on a chain of optical amplifiers to boost the lightwave signal power to a level sufficient for the signals to traverse the entire system. Such lightwave systems are carefully specified to have sufficient amplifier gain, proper amplifier output power, and correct amplifier compression. The system parameters mentioned above, together with all other system parameters, ensure that the resulting lightwave communication system operates as expected, usually, in accordance with a desired signal-to-noise ratio performance. Of course, it is the signal-to-noise performance of a lightwave communication system which is affected by such factors as amplifier gain, output power, and compression.
Amplifier gain is generally set at an appropriate level for compensating effectively intrinsic loss of optical fiber in the span between two adjacent amplifiers. Amplifier output power is usually established between maximum and minimum power levels. The maximum power level is that level below which nonlinear effects in the optical fiber do not significantly distort the lightwave signals; the minimum power level is that level required to traverse the optical fiber span between amplifiers while maintaining an adequate signal-to-noise ratio. Compression is the amplifier parameter which determines the degree to which the amplifier operates in saturation. When a chain of amplifiers in a lightwave system are operated in compression, the system is self-stabilizing with respect to slight signal power or amplifier fluctuations.
In present lightwave communication systems, specified amplifier output power, gain, and compression prohibit the use of sufficient amplifier pump power to effectively and uniformly invert the amplifier gain medium. This, in turn, gives rise amplifier element designs and loss element designs to meet the prescribed criteria of gain, output power, and compression for the arrangement.
In an illustrative embodiment, a doped-fiber amplifier is optically coupled to a loss element to realize the optical arrangement. Counter-propagating or copropagating pump signals or a combination of both types of pump signals are employed for pumping the amplifier.